Loop antenna



u y 1952 A. c. STELZER 2,602,891

LOOP ANTENNA Filed May 11, 1950 INVEN TOR.

#40/20/755 Mar/95 jze/za/ QZOQW 4,204 mm 45 A TTO/F/VEYS Patented July 8, 1952 LOOP ANTENNA Alphonse Charles Stelzer, Mount Healthy, Cincinnati, Ohio, assignor to Avco Manufacturing Corporation, Cincinnati, Ohio, a corporation of Delaware Application May 11, 1950, Serial No. 161,436

1 Claim. 1

The present invention relates to loop antennas and, without limiting the generality of the foregoing, particularly to the type employed in portable radio receivers and more specifically to a novel loop antenna the L of which is critically predetermined, and to a novel process for making the same.

In many radio receivers in which a loop antenna is included as a frequency-determining parameter in the antenna input circuit, which circuit is tuned as by a gang capacitor in tracking relationship to a local oscillator tank circuit, it is necessary to maintain the inductance (L) of the loop antenna within very close tolerances. In one well-known receiver of this type the first tube following the antenna is a pentagrid converter. A loop antenna, paralleled by a variable section of a gang condenser, is R. F. coupled between the third grid and cathode of this tube; and an oscillator tank circuit, tuned by another variable section of this gang condenser, and a feedback coil are associated with various electrodes of the tube in a known manner. The two variable capacitor sections are ganged together. In receivers of this and similar types it is necessary to maintain the predetermined desired L of the loop within very close tolerances, and the present invention provides a novel loop in which this objective is achieved and a novel process for accomplishing it.

An object of the invention is to provide a loop antenna construction which precisely and rigidly adheres to a predetermined desired L characteristic.

Another fundamental object of the invention is to provide a novel process for adjusting the L of a loop within a narrow range of tolerances.

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following description of the accompanying drawings, in which there is shown a. preferred illustrative form of loop antenna made in accordance with the present invention. In the drawings:

Fig. l is an illustration of a top view of a loop antenna in accordance with the invention;

Fig. 2 is a front view thereof; and

Figs. 3 and 4 are sectional views taken on lines 3-3 and 4-4 of Fig. 1, looking in the direction of the arrows.

The loop is made of multistrand Litz wire wound by conventional machinery intothe desired loop configuration illustrated in Fig. 1, each successive turn of the loop being located radially outwardly from the preceding one so that the thickness of the loop is that of the single conductor. The wire is suitably insulated as by a thin plastic covering.

Preparatory to the winding process a suitable adhesive is applied to the conductor for purposes of securing the adjacent loop turns together. After drying the rigid loop is placed between two thin plastic Celluloid protective mounting members H and I2, each of which is slightly larger than the loop in area and is formed with a generally semicircular configuration to'provide protective margins extending outwardly beyond the periphery of the loop. Each of the mounting members is preformed with a plurality of large apertures such as those indicated by the reference numerals l3, l4, l5, and I6, for a purpose to be explained hereinafter. Each of the mounting members is also preformed with suitable apertures to receive the terminal eyelets l1 and [8 to which the leads l9 and 20 are brought out. The mounting members are also provided with preformed small apertures 2i, 22, 23, and 24, whereby the loop assembly may be secured to the lid of a portable radio receiver cabinet, for example, by conventional means. The assembly of loop and dielectric mounting members is secured together by strips of adhesive tape 25, 26, 21 interlocking with notches 28, 29, and 30 formed on the mounting members. It will be observed that the mounting members are identical and the same die may be employed to punch them out from the plastic dielectric material of which they are made.

In practical commercial operations, as will be clearly understood by those skilled in the art, it is customary to manufacture the loop as a complete component. Before the loop is sent to the assembly line for incorporation in a radio receiver it is assumed that the inductance of the loop falls within predetermined tolerances. In receiving circuits in which the loop is a substantial frequency-determining parameter, it is necessary to maintain the inductance within very close tolerances. It will be appreciated that, once the loop [0 is formed, it would be a difficult, cumbersome, expensive, and time-consuming operation to add or to subtract turns or to rearrange the leads or to separate the loop from the mounting assembly preparatory to final testing of the loop and measurement of its inductance. Nonetheless, those are the expedients that have heretofore been employed. In accordance with the present invention, there is provided a process by which these time-consuming and expensive operations are dispensed with and more precise quantitative determination of the L of the loop is accomplished.

tained, a peaking condition being indicated by an R. F. voltmeter. Since the resonant condition is a mathematical function of the inductance and capacitance parameters in the circuit. the frequency of the applied signal being constant, the capacitor can readily be calibrated in terms of inductance. I

Let it be assumed for purposes of discussion that the inductance of an assembly of this type, after measurement by an L-meter, is found to be too low. In accordance with the invention the operator continues to observe the indications of the meter while painting onto the loop a molten mixture comprising powdered iron and impregnating wax. It will be observed that sections of the loop are in registry with apertures it, Hi, l5, and I5, whereby access is permitted for that purpose. an ordinary commercially available varnish or paint brush. The deposit of the metallic magnetic material increases the L of the loop, and the operator simply continues the brushing operation and the depositing of such material until the a meter reading reaches the desired L value.

Although I do not desire to be limited to any specific proportions, or to any specific powdered magnetic material, satisfactory results have been achieved with a mixture of impregnating wax and iron powder, the proportion of the latter being to by volume, and the temperature of the molten mixture being maintained at to degrees centigrade. The iron powder has a tendency to settle to the bottom of the container, and it is desirable to immerse the applicator deeply therein or to agitate the mixture frequently. Any commercially available impregnating wax such as ceresine is entirely suitable. Any suitable substance which will not destroy the insulation on the loop conductor, which solidifies at ordinary temperatures encountered in operation and has a low melting point may be used in lieu of the preferred wax. Any high permeability iron or ferromagnetic powder will serve as the powder component. Many suitable materials and alloys are known to the art. Low magnetic retentivity is desired.

The application is made by the use of 7 The outstanding advantage of this process is that the ferromagnetic material which increases the inductance of the loop is deposited in increments well within the control of a relatively unskilled operator, while the meter reading is observed so that there is no necessity to add or subtract turns or to disturb the loop leads or the mounting assembly in order to make the final adjustment to the desired inductance value. After deposition the molten mixture solidifies into durable masses 3|, 32 strongly adhering to the loop proper, and the assembly maintains the desired inductance characteristic throughout the usable life of commercially available radio receivers.

In mass production of loops in accordance with the present invention, an endeavor is made to keep the desired inductance value within or below the specified limits at the end of the winding operation, and special precautions are taken to prevent those values from exceeding such limits. Therefore only those loops which do not conform to the specified inductance value are subjected to the process in accordance with the invention, such process comprising a method for increasing the inductance of a loop.

While there hasbeen shown and described what is at present regarded as the preferred embodiment of the invention, it will be understood by those skilled in the art that various modifications and substitutions of equivalents may be made therein without departing from the spirit and scope of the invention as defined by the claim hereto appended.

Having fully discussed and described my invention, I claim: 7

A loop antenna having a predetermined induct ance comprising a pair of thin dielectric moun ing members formed with spaced apertures therein, a fiat conductive antenna loop comprising a plurality of turns of insulated wire, said loop bein so formed as to have a portion in registry with at least one of said apertures, and an adhering magnetic mixture adjacent only those portions of said loop in registry with said apertures.

ALPI-IONSE CHARLES STELZER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,961,667 Johnson Dec. 10, 1930 2,133,301 Martin Oct. 18, 1938 2,329,634 McDonald Sept. 14, 1943 2,457,806 Crippa Jan. l, 1949 2,460,492 Cutler et al. Feb. 1, 1949 

